Re: [PATCH v12 7/9] cgroup/cpuset: Fail if isolated and nohz_full don't leave any housekeeping

[Waiman Long <llong@redhat.com>]

On 9/15/25 10:59 AM, Gabriele Monaco wrote:
> Currently the user can set up isolated cpus via cpuset and nohz_full in
> such a way that leaves no housekeeping CPU (i.e. no CPU that is neither
> domain isolated nor nohz full). This can be a problem for other
> subsystems (e.g. the timer wheel imgration).
>
> Prevent this configuration by blocking any assignation that would cause
> the union of domain isolated cpus and nohz_full to covers all CPUs.
>
> Acked-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
> ---
>   kernel/cgroup/cpuset.c | 63 ++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 63 insertions(+)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 81a9239053a7..3cedc3580373 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1275,6 +1275,19 @@ static void isolated_cpus_update(int old_prs, int new_prs, struct cpumask *xcpus
>   		cpumask_andnot(isolated_cpus, isolated_cpus, xcpus);
>   }
>   
> +/*
> + * isolated_cpus_should_update - Returns if the isolated_cpus mask needs update
> + * @prs: new or old partition_root_state
> + * @parent: parent cpuset
> + * Return: true if isolated_cpus needs modification, false otherwise
> + */
> +static bool isolated_cpus_should_update(int prs, struct cpuset *parent)
> +{
> +	if (!parent)
> +		parent = &top_cpuset;
> +	return prs != parent->partition_root_state;
> +}
> +
>   /*
>    * partition_xcpus_add - Add new exclusive CPUs to partition
>    * @new_prs: new partition_root_state
> @@ -1339,6 +1352,42 @@ static bool partition_xcpus_del(int old_prs, struct cpuset *parent,
>   	return isolcpus_updated;
>   }
>   
> +/*
> + * isolated_cpus_can_update - check for isolated & nohz_full conflicts
> + * @add_cpus: cpu mask for cpus that are going to be isolated
> + * @del_cpus: cpu mask for cpus that are no longer isolated, can be NULL
> + * Return: false if there is conflict, true otherwise
> + *
> + * If nohz_full is enabled and we have isolated CPUs, their combination must
> + * still leave housekeeping CPUs.
> + */
> +static bool isolated_cpus_can_update(struct cpumask *add_cpus,
> +				     struct cpumask *del_cpus)
> +{
> +	cpumask_var_t full_hk_cpus;
> +	int res = true;
> +
> +	if (!housekeeping_enabled(HK_TYPE_KERNEL_NOISE))
> +		return true;
> +
> +	if (del_cpus && cpumask_weight_and(del_cpus,
> +			housekeeping_cpumask(HK_TYPE_KERNEL_NOISE)))
> +		return true;
> +
> +	if (!alloc_cpumask_var(&full_hk_cpus, GFP_KERNEL))
> +		return false;
> +
> +	cpumask_and(full_hk_cpus, housekeeping_cpumask(HK_TYPE_KERNEL_NOISE),
> +		    housekeeping_cpumask(HK_TYPE_DOMAIN));
> +	cpumask_andnot(full_hk_cpus, full_hk_cpus, isolated_cpus);
> +	cpumask_and(full_hk_cpus, full_hk_cpus, cpu_active_mask);
> +	if (!cpumask_weight_andnot(full_hk_cpus, add_cpus))
> +		res = false;
> +
> +	free_cpumask_var(full_hk_cpus);
> +	return res;
> +}
> +
>   static void update_exclusion_cpumasks(bool isolcpus_updated)
>   {
>   	int ret;
> @@ -1464,6 +1513,9 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
>   	if (!cpumask_intersects(tmp->new_cpus, cpu_active_mask) ||
>   	    cpumask_subset(top_cpuset.effective_cpus, tmp->new_cpus))
>   		return PERR_INVCPUS;
> +	if (isolated_cpus_should_update(new_prs, NULL) &&
> +	    !isolated_cpus_can_update(tmp->new_cpus, NULL))
> +		return PERR_HKEEPING;
>   
>   	spin_lock_irq(&callback_lock);
>   	isolcpus_updated = partition_xcpus_add(new_prs, NULL, tmp->new_cpus);
> @@ -1563,6 +1615,9 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
>   		else if (cpumask_intersects(tmp->addmask, subpartitions_cpus) ||
>   			 cpumask_subset(top_cpuset.effective_cpus, tmp->addmask))
>   			cs->prs_err = PERR_NOCPUS;
> +		else if (isolated_cpus_should_update(prs, NULL) &&
> +			 !isolated_cpus_can_update(tmp->addmask, tmp->delmask))
> +			cs->prs_err = PERR_HKEEPING;
>   		if (cs->prs_err)
>   			goto invalidate;
>   	}
> @@ -1914,6 +1969,12 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
>   			return err;
>   	}
>   
> +	if (deleting && isolated_cpus_should_update(new_prs, parent) &&
> +	    !isolated_cpus_can_update(tmp->delmask, tmp->addmask)) {
> +		cs->prs_err = PERR_HKEEPING;
> +		return PERR_HKEEPING;
> +	}
> +
>   	/*
>   	 * Change the parent's effective_cpus & effective_xcpus (top cpuset
>   	 * only).
> @@ -2934,6 +2995,8 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>   		 * Need to update isolated_cpus.
>   		 */
>   		isolcpus_updated = true;
> +		if (!isolated_cpus_can_update(cs->effective_xcpus, NULL))
> +			err = PERR_HKEEPING;
>   	} else {
>   		/*
>   		 * Switching back to member is always allowed even if it
Reviewed-by: Waiman Long <longman@redhat.com>